1. Field of the Invention
The invention relates to the structure of a read sensor used in a hard disk drive for magnetic recording. Specifically, the invention relates to a current-perpendicular-to-plane (CPP) tunneling magnetoresistance (TMR) read sensor with dual sense layers that exhibit a negative saturation magnetorestriction.
2. Description of the Related Art
In the data reading process of a hard disk drive, a read head passes over magnetic transitions of a data track on a rotating hard disk, and magnetic fields emitting from the magnetic transitions modulate the resistance of a read sensor in the read head. Changes in the resistance of the read sensor are detected by a sense current passing through the read sensor. The resistance changes can be read directly and converted into voltage signals from which read data encoded in the magnetic transitions of the data track are generated. Two of the most common read sensors used in modern hard disk drives are a current-perpendicular-to-plane (CPP) tunneling magnetoresistance (TMR) read sensor, and a CPP giant magnetoresistance (GMR) read sensor. The TMR read sensor consists of an electrically insulating MgOx barrier layer sandwiched between lower and upper sensor stacks, and relies on an electron tunneling effect across the barrier layer to generate the resistance changes. The GMR read sensor consists of an electrically conducting copper (Cu) or copper oxide (Cu—O) spacer layer sandwiched between the lower and upper sensor stacks, and relies on an electron scattering effect at the spacer layer to generate the resistance changes. The subject of this disclosure is focused on the TMR read sensor.
FIG. 1 (Prior Art) is a partial cross sectional view of an assembly of read and write heads 100. A read head 104 is combined with a write head 102 in a longitudinal type to form the assembly of read and write heads 100. As will be recognized by those skilled in the art, the write head 102 may also be a perpendicular type as well. In the read head 104, a TMR read sensor 106 is sandwiched between a lower shield 108 and an upper shield 110. In the write head 102, a lower pole 112 and an upper pole 116 are separated by a write gap 114, but are connected at a backgap 122. The write head 102 also comprises a yoke 120 and coils 118.
The assembly of read and write heads is supported by a slider that is mounted on a suspension arm. When the hard disk rotates, an actuator swings the suspension arm to place the slider over selected circular data tracks on the hard disk. The suspension arm biases the slider toward the hard disk, and an air flow generated by the rotation of the hard disk causes the slider to fly on a cushion of air at a very low fly height over the hard disk. When the slider rides on the air, the air bearing surface (ABS) of the read and write heads faces the air, and the actuator moves the suspension arm to position the read and write heads over selected data tracks on the hard disk. The read and write heads read data from and write data to, respectively, data tracks on the hard disk.
FIG. 2 (Prior Art) is an ABS view of a read head 200 (equivalent to the read head 104 of FIG. 1). The read head 200 includes the TMR sensor 106, which is sandwiched between the lower shield 108 and the upper shield 110, and is separated by side oxide layers 220 from longitudinal bias layers 222. The TMR read sensor 106 consists of a barrier layer 206, which is sandwiched between a lower sensor stack and an upper sensor stack. The barrier layer 206 typically comprises an electrically insulating MgOx film.
A typical lower sensor stack consists of a buffer layer 218 comprising a nonmagnetic Ta film, a seed layer 216 comprising a nonmagnetic Ru film, a pinning layer 214 comprising an antiferromagnetic Ir—Mn film, a keeper layer 212 comprising a ferromagnetic Co—Fe film, an antiparallel coupling layer 210 comprising a nonmagnetic Ru film, and a reference layer 208 comprising a ferromagnetic Co—Fe—B film. The keeper, antiparallel-coupling and reference layers form a flux-closure structure where four fields are induced. First, a unidirectional anisotropy field (HUA) is induced by exchange coupling between the pinning layer and the keeper layer. Second, a bidirectional anisotropy field (HBA) is induced by antiparallel coupling between the keeper and reference layers and across the antiparallel-coupling layer. Third, a demagnetizing field (HD) is induced by the net magnetization of the keeper and reference layers. Fourth, a ferromagnetic-coupling field (HF) is induced by ferromagnetic coupling between the reference and sense layers and across the barrier layer. To ensure proper sensor operation, HUA and HBA should be high enough to rigidly pin magnetizations of the keeper and reference layers in opposite transverse directions perpendicular to the ABS, while HD and HF should be small and balance with each other to orient the magnetization of the sense layer in a longitudinal direction parallel to the ABS.
A typical upper sensor stack consists of a sense layer 204 comprising a ferromagnetic Co—Fe—B film and a cap layer 202 comprising a nonmagnetic Ru film. Both the Co—Fe—B reference and sense layers exhibit a “soft” amorphous phase after depositions, which will be transformed into a polycrystalline phase after annealing. With this crystallization, a Co—Fe—B(001)[110]//MgOx(001)[100]//Co—Fe—B(001)[110] epitaxial relationship is developed, and thus coherent spin polarization through the MgOx barrier layer is induced, thereby enhancing a TMR effect.
To ensure stable sensor operation, it is desirable that the sense layer exhibits a negative, or at least zero, saturation magnetostriction (λS). After receiving compressive stresses induced by mechanical lapping in the fabrication process of the read and write heads, a sense layer with a negative λS will longitudinally bias its own magnetization in a longitudinal direction parallel to the ABS in the absence of an external magnetic field. Such a sense layer only requires low longitudinal bias fields provided by neighboring longitudinal bias layers for the stable read performance. In contrast, a sense layer with a positive λS may bias its own magnetization in a transverse direction perpendicular to the ABS in the absence of an external magnetic field. This sense layer thus requires high longitudinal bias fields for the stable read performance. The increase in the longitudinal bias fields will lead to a decrease in read sensitivity. In addition to attaining a desirable negative λS for the sense layer, it is crucial for the TMR sensor to maintain a high TMR coefficient (ΔRT/RJ) at a low junction resistance-area product (RJAJ), where RJ is a minimum junction resistance measured when the magnetizations of the reference and sense layers are parallel to each other, and RJ+ΔRT is a maximum junction resistance measured when the magnetizations of the reference and sense layers are antiparallel to each other, and AJ is a junction area.
In the prior art, the most extensively explored TMR read sensor with reference and sense layers comprising 60Co-20Fe-20B (in atomic percent) films exhibits ΔRT/RJ of as high as 138% at RJAJ of as low as 2.4 Ω-μm2, after annealing for 2 hours at 360° C. in 8,000 Oe in a high vacuum oven. However, its sense layer exhibits λS of as high as +6×10−6. This high λS originates from the high Fe content and the high-temperature annealing. A conventional approach of adding a second sense layer comprising a Ni—Fe film does produce a negative λS, but at the expense of decreasing ΔRT/RJ to an unacceptably low value of 40% and increasing RAJ to an unacceptably high value of 4 Ω-μm2. Such deteriorated TMR properties originates from unwanted Ni diffusion from the Ni—Fe to Co—Fe—B sense layers. Preferably, it is desirable to have a TMR read sensor with a negative λS, combined with ΔRT/RJ of greater than 80% at RJAJ of less than 2 Ω-μm2.
What is needed is attaining a negative λS for the sense layer while maintaining high ΔRT/RJ at low RAJ for the TMR read sensor.